The present invention relates to a semiconductor absolute pressure transducer, and particularly to method for bonding a covering member of insulating material on a surface of a silicon diaphragm assembly on which piezoresistive elements and conducting paths are constructed.
In U.S. Pat. Nos. 3,918,019 and 4,079,508, there is shown a transducer assembly having a glass substrate and a thin silicon diaphragm upon which is diffused a piezoresistive bridge circuit. Bridge circuit components are properly oriented and connected to bonding pads and conducting paths, both of which are formed on the silicon. The glass substrate has a circular well formed therein having a diameter at least as large as the diameter of the diaphragm. Conducting leads are deposited on the glass substrate in a pattern matching that of the bonding pads on the silicon. The silicon is bonded to the glass substrate with the silicon diaphragm overlying the well in the glass and the bonding pads overlying the conducting leads deposited on the glass by Anodic Bonding method. The bond formed by the method provides a hermetic seal around the well.
Ordinarily, a layer of insulating material such as silicon dioxide (SiO.sub.2) is provided on the surface of the silicon for protecting the piezoresistive bridge circuit and the conducting paths formed thereon. In such case, it is very difficult to bond an insulating member, for example the glass substrate, on the surface of the silicon firmly, by the method such as Anodic Bonding. Particularly, this is true when the silicon oxide (SiO.sub.2) layer is formed with a thickness of more than 0.5 .mu.m.
In U.S. Pat. No. 3,595,719, a method of bonding an insulator member to a passivating layer covering a surface of a semiconductor is described. In this method, etching of the passivating layer to a thickness of at least about 1,000 A. with an etchant is performed prior the Anodic Bonding.
If such method is adapted for manufacturing of the transducer assembly mentioned above, the characteristic of the piezoresistive bridge circuit becomes worse, because of a high voltage applied across the p-n juncture thereof. Namely, the silicon diaphragm assembly is n-type and the pressure sensitive resistors of the piezoresistive bridge circuit which are diffused thereon are p-type. When a positive voltage is applied to a silicon and the negative voltage to the glass substrate, leak current flows through the p-n juncture, thereby, affecting the insulating characteristic of the p-n juncture.
Further, the conducting paths are formed by p+ diffusion which yields a low resistivity. However, this p+ diffusion also causes grooves along the paths, though they are very shallow in the depth. And the grooves have influence upon the hermetic seal around the well. In U.S. Pat. Nos. 3,918,019 and 4,079,508, the bonding pads of gold are provided for the hermetic seal.
Another prior art is as follows:
(1) U.S. Pat. No. 3,397,278, "ANODIC BONDING".
This relates to an Anodic Bonding technique.